1. Field of the Invention
This invention relates to a process for preparing 2-amino-1-butanol from the L-tartarate of 2-amino-1-butanol.
It is well known that 2-amino-1-butanol is industrially advantageous as an intermediate of ethanbutol[2,2'-ethylendiimino)-di-1-butanol] which is valuable for use in medicines, e.g., as disclosed in British Pat. No. 961,317.
Having an asymmetric carbon, 2-amino-1-butanol has two optical isomers, i.e., the dextro-form (d) and the levoform (l). Of the two ethanbutols synthesized from the aforesaid two optical isomers, however, only the ethanbutol synthesized from d-2-amino-1-butanol has a pharmacological effect, and the ethanbutol synthesized from l-2-amino-1-butanol not only is pharmacologically ineffectual, but also has undesirable side effects.
Therefore, it is necessary to separate d- from l-2-amino-1-butanol by optical resolution. The d-2-amino-1-butanol thus separated can be used as an intermediate of ethanbutol; on the other hand, the l-2-amino-1-butanol thus separated can be used as a starting material for optical resolution, after racemization.
2. Description of the Prior Art
As for the method of the optical resolution mentioned above, a well known method comprises reacting d- and l-2-amino-1-butanol with L-tartarate to form diastereoisomers, which are then optically resolved by recrystallization in an aqueous solvent, such as water or an aqueous solution of an organic solvent such as an alcohol in water solution, etc. Then the obtained crystal of L-tartarate of d-2-amino-1-butanol are reacted with an alkaline earth metal compound in an aqueous solvent to precipitate an L-tartarate of the alkaline earth metal compound. Free d-2-amino-1-butanol is recovered in the solvent.
According to our research, however, in the reaction of the L-tartarate of d-2-amino-1-butanol or l-2-amino-1-butanol with an alkaline earth metal compound in an aqueous solvent, e.g., water, when the aforesaid two components are mixed in the conventional way, e.g., by random charging, the obtained crystals of the L-tartarate of the alkaline earth metal compound have small diameters and poor filterability. In addition, the filter cake obtained has a high water content and moreover presents difficulties in rinsing. Therefore, one is unable to maintain the yield of d-2-amino-1-butanol or l-2-amino-1-butanol at high levels with ease. It is desirable to form the precipitates which don't contain a large amount of small crystals having dimensions less than 15.mu..times. 15.mu. .
By the present invention, however, and taking the method of mixing into special consideration, gross crystals of the L-tartarate of the alkaline earth metal compound can be obtained and d- or l-2-amino-1-butanol can be obtained in high yields with ease.